The present invention relates to an actuator for raising or moving a variable or fixed load. The invention relates more particularly to an actuator designed to minimize the power required to operate it and to have a simple means of controlling with great accuracy the speed and position of the load controlled by the actuator.
Actuators used to move large loads must be rated to support the maximum load that will be applied to them, which leads to the use of high-power actuators, without all of that power being indispensable at all times.
A plurality of actuators can be used, depending on the size of the load, its disposition, or the movements to be imparted to the load. In the case of large masses, load compensation is applied, for example using a counterweight or a mechanical spring. Precise movement of a load by a plurality of actuators necessitates delicate synchronization of the actuators.
European Patent EP-A-0070811 (SELENIA INDUSTRIE) describes a hydraulic actuator supplied by a pump and whose hydraulic circuit includes distributor valves. The actuator is provided with a mechanical hydraulically or manually operated safety system. The piston of the actuator includes a cavity in which a reversible screw-and-nut system is accommodated. The screw is constrained to move with the piston in translation and in rotation. The nut is mounted to rotate in the body of the actuator, but it can be held by a locking finger that is active by default, unlocked manually or hydraulically, and acts on the outside portion of a free-wheel fastened to the nut.
Extending the actuator causes the reversible screw-and-nut system to rotate the nut in the direction that is always permitted by the free-wheel. On the other hand, retracting the actuator rotates the nut in the direction in which it rotates the outside portion of the free-wheel, with the result that the piston can be retracted only if the locking finger that acts on the outside portion of the free-wheel is unlocked. Because the locking finger is de-activated manually or hydraulically, this guarantees that the retraction of the actuator is intentional, and is not due to a loss of hydraulic power.
The hydraulic pump must be rated for the maximum load that the actuator must support. It will therefore be overrated for many uses.
The accuracy of the movement of the actuator depends on the solenoid valve and the pump. For some uses, such as moving a large load with great accuracy, for example for an assembly operation or when using several actuators simultaneously to lift a load accurately, these systems are not sufficiently accurate, because it is difficult to control their position and to synchronize them.
The hydraulic power supply circuit, which includes a hydraulic pump, a solenoid valve, an oil reservoir and several hydraulic pipes, is both bulky and costly, and can cause breakdowns that can cause the hydraulic power supply to fail.
It may be desirable to hold the actuator in a precise position and under a particular load for a long period of time. It is then advantageous not to have to supply energy to hold the actuator in that precise position. The actuator described in European Patent EP-A-0070811 (SELENIA INDUSTRIE) can be locked mechanically. The necessary accuracy cannot be obtained, however. The locking finger acts on the outside portion of a toothed free-wheel. The locking positions are therefore discrete, not infinite.